Raman shifting devices include a Raman scattering medium, usually a high pressure gas cell. In such devices laser radiation at a given wavelength is converted to a radiation which has a longer wavelength by a process of Stimulated Raman scattering. This longer wavelength radiation is termed stokes Radiation or Raman-shifted Radiation. The scattering medium is chosen for conversion of the laser radiation to the desired wavelength. By using methane which has a frequency shift of 2916 cm.sup.-1 it is possible to convert the 1.06 micron radiation of the Neodymium:YAG laser to 1.54 micron which is an "eyesafe" wavelength.
Several conversion schemes are known in the prior art. For example in U.S. Pat. No. 3,668,420 issued to Vanderslice in 1972, the Raman converter used includes two mirrors which form a resonator for the shifted wavelength. In European Patent No. 63205, Bess and Ruger (1985), a single mirror is used to form a "semi resonator". A more comprehensive description is given by Parazzoli et al in IEEE Journal of Quantum Electronics Vol. 24 (1988) p. 872. In both of these designs the mirrors used must be aligned with respect to the pump source radiation in order to reflect the wavelength-shifted radiation to the region in which the pump source radiation is focused. This alignment is critical for the suppression of the competing process of stimulated Brillouin scattering and must be precisely maintained thereby complicating the mechanical construction of the converter.
In an attempt to overcome this drawback, Bruesselbach and Whirst described in their international patent application WO 86/02784 a design in which the mirror of the Raman semi-resonator is also the output mirror of the laser source. This design does not guarantee however an automatic alignment of the Raman converter since the laser beam may deviate from the normal to the output mirror as a result of thermal effects in the active medium or in the Q-switching material or due to misalignment of the laser mirrors.